1. Field of the Invention
The present invention relates to a control apparatus for a motor-driven power steering system which is adopted in automobiles or motor vehicles for assisting a driver in manipulating a steering wheel by using an electric motor.
2. Description of Related Art
As a means for securing safety upon occurrence of a failure in a controller implemented by a microcomputer or the like for controlling a motor-driven power steering system of a motor vehicle, there is known an apparatus in which match or mismatch between a direction of steering torque and a driving direction of an electric motor is detected, and when the mismatch is detected, a time period during which the mismatch continues to exist is measured, wherein operation of the driving motor is disabled or inhibited in case the time period mentioned above exceeds a predetermined value, as is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 137651/1995 (JP-A-7-137651).
For having better understanding of the concept of the invention, technical background thereof will be elucidated below in some detail. FIG. 19 is a schematic block diagram showing a general arrangement of a motor-driven power steering system set forth in claim 1 of the aforementioned Japanese Unexamined Patent Application Publication No. 137651/1995 (JP-A-7-137651). Referring to the figure, the motor-driven power steering system is comprised of a steering torque detecting unit 1 for detecting a steering torque applied to a steering wheel by a driver, an electric motor 2 for generating an assist torque for aiding the driver in his or her steering operation, a CPU 3 serving as a control means for outputting a motor drive signal for controlling the motor 2 in dependence on the output of the steering torque detecting unit 1, a clockwise/counterclockwise drive inhibit region decision means 13 for deciding that the output of the steering torque detecting unit 1 lies in a region for inhibiting the driving operation of the electric motor in the clockwise direction or counterclockwise direction, a logical match/mismatch verifying circuit 14 for logically deciding whether the direction of the steering torque matches or mismatches the driving direction of the motor 2 on the basis of the output of the clockwise/counterclockwise drive inhibit region decision means 13 and a motor driving direction command signal issued by the CPU 3, a timer 15 for measuring a time duration or period for which the logical mismatch state detected by the logical match/mismatch verifying circuit 14 continues to exist, an inhibiting/enabling circuit 16 for disabling or inhibiting the driving operation of the motor 2 when the time period measured by the timer 15 exceeds a predetermined value, and a motor drive means 6 for driving the motor 2 in accordance with a motor driving signal issued by the CPU 3 when driving of the motor 2 is not inhibited by the inhibiting/enabling circuit 16 while disabling the driving operation of the motor when the inhibiting/enabling circuit 16 outputs a signal inhibiting the driving operation of the motor 2.
Further, the motor-driven power steering system mentioned above may include a motor current detecting means 4 for detecting a current flowing through the motor 2, a dead band detecting circuit 17 for detecting that the steering torque lies within a dead band, and a turn-back current level decision circuit 18 for deciding whether or not the current flowing through the motor 2 upon turning back the steering wheel is excessively large when the detected value outputted from the current detecting means 4 lies within the dead band, as set forth in claim 4 of the aforementioned application.
Now, description will be directed to the hitherto known motor-driven power steering system implemented in the structure mentioned above.
In general, the motor-driven power steering system is designed to drive the electric motor in a direction for reducing the steering effort of the driver in dependence on the steering torque applied to the steering wheel by the driver by increasing the motor current and hence the assist torque as the steering torque increases. Besides, the motor-driven power steering system is so arranged that the control value for the motor current for a given steering torque can be changed or modified in dependence on the speed of the motor vehicle.
Let's assume, by way of example, that a driver rotates slowly the steering wheel in one direction, starting from a neutral point. In that case, the direction of the steering torque coincides with the driving direction of the motor 2. Consequently, output of the logical match/mismatch verifying circuit 14 indicates logical match. Accordingly, the timer 15 is not activated with the driving operation of the motor 2 being enabled by the inhibiting/enabling circuit 16. Thus, the motor 2 is driven in accordance with the motor driving signal outputted from the CPU 3.
On the other hand, when the steering wheel is turned back to the neutral point after the steering operation by driving the motor 2 in the state in which the steering torque lies within the dead band, the output of the logical match/mismatch verifying circuit 14 indicates logical mismatch. Consequently, the timer 15 is activated. After lapse of a time preset at the timer 15, the inhibiting/enabling circuit 16 inhibits the driving operation of the motor 2. However, by selecting the time set at the timer 15 to be longer than the time for which the motor 2 is driven in the dead band or alternatively selecting the time for driving the motor 2 shorter than the time set at the timer 15, the driving operation of the motor 2 is not inhibited by the inhibiting/enabling circuit 16. In that case, the motor 2 can be driven in accordance with the motor driving signal outputted from the CPU 3.
When a failure or abnormality takes place in the controller or CPU 3, the motor-driven power steering system operates such that a motor driving signal of magnitude which allows self-rotation of the steering wheel is outputted continuously independent of the steering torque. In general, three operation modes 1), 2) and 3) mentioned below of the motor-driven power steering system may be conceived.
1) When the driver is manipulating the steering wheel with his or her hands and when the direction of the steering torque coincides with the driving direction of the motor 2, the output of the logical match/mismatch verifying circuit 14 indicates logical match. Consequently, the timer 15 is not activated while the inhibiting/enabling circuit 16 enables the driving operation of the motor 2. Thus, the steering operation can be aided by the assist torque generated by the motor 2.
2) By contrast, when the driver manipulates the steering wheel and when the direction of the steering torque does not coincide with that of the driving direction of the motor 2, then the output of the logical match/mismatch verifying circuit 14 indicates logical mismatch, as a result of which the timer 15 is activated. Before the time set at the timer 15 elapses, the inhibiting/enabling circuit 16 does not inhibit the driving operation of the motor 2. Thus, the motor 2 generates a torque in the direction opposite to the steering direction. Consequently, load encountered in the steering operation increases. On the other hand, after the time set at the timer 15 has lapsed, the driving operation of the motor 2 is no more inhibited by the inhibiting/enabling circuit 16. Thus, the motor 2 stops generation of the torque. Consequently, the steering wheel can be manipulated only manually.
3) When the steering torque lies within the dead band with the driver releasing the steering wheel, the output of the logical match/mismatch verifying circuit 14 indicates logical mismatch, triggering operation of the timer 15. Before the time set at the timer 15 elapses, the inhibiting/enabling circuit 16 does not inhibit the driving operation of the motor 2. Consequently, the motor 2 generates a torque for forcing the steering wheel to rotate by itself (i.e., self-rotation without being manipulated by the driver). However, after the time set at the timer 15 has lapsed, the inhibiting/enabling circuit 16 inhibits driving operation of the motor 2 which can then generate no torque. Consequently, by selecting the time set at the timer 15 to be adequately short, self-rotation of the steering wheel (i.e., rotation of the steering wheel in the released state of the steering wheel) can be prevented.
Further, in the motor-driven power steering system equipped with the motor current detecting means 4, the dead band detecting circuit 17 and the turn-back current level decision circuit 18, the state in which the steering torque lies within the dead band is detected by the dead band detecting circuit 17. When the steering torque lies within the dead band and when the detected value of the motor current as detected by the motor current detecting means 4 is smaller than a predetermined value, the timer 15 is reset by the turn-back current level decision circuit 18, whereby the temporal restriction imposed onto the driving operation of the motor 2 in the dead band is cleared.
The conventional motor-driven power steering system described above suffers a problem that the driving operation of the motor 2 can be inhibited only with a delay from occurrence of abnormality in the CPU 3 because the driving operation of the motor 2 is inhibited or disabled after lapse of the time preset at the timer 15. In particular, when the direction of the steering torque does not coincide with the driving direction of the motor 2 and when the time preset at the timer 15 does not yet lapsed, a torque of the direction reverse to the steering direction is generated by the motor 2, incurring a problem that the steering operation encounters heavy load.
Additionally, in the case of the conventional motor-driven power steering system which is not equipped with the motor current detecting means 4, the dead band detecting circuit 17 and the turn-back current level decision circuit 18, the control for turning back the steering wheel toward the neutral point after the steering operation will encounter difficulty when the time preset at the timer 15 is selected short. Namely, when the motor 2 is to be driven continuously in the state where the steering torque lies within the dead band, the driving operation of the motor 2 is inhibited by the inhibiting/enabling circuit 16, giving rise to another problem. On the other hand, when the preset time is selected long, the motor driving signal for allowing self-rotation of the steering wheel is continuously outputted regardless of the steering torque when abnormality takes place in the CPU 3. Consequently, the time taken for the driving operation of the motor 2 to be inhibited by the inhibiting/enabling circuit 16 becomes long, to another disadvantage.
In the case of the motor-driven power steering system equipped with the motor current detecting means 4, the dead band detecting circuit 17 and the turn-back current level decision circuit 18, there is no necessity for limiting temporally the driving operation of the motor 2 in the dead band, e.g. in the turn-back steering. However, the structure of the motor-driven power steering system is much complicated with the number of components being increased, presenting a problem that the cost involved in manufacturing the motor-driven power steering system increases.